This invention relates to a center valve for a master cylinder used as a hydraulic pressure source for a vehicle hydraulic brake system or a hydraulic clutch.
FIG. 1 shows a master cylinder embodying the present invention. It includes a cylinder body 2 having a closed end. A piston 3 is inserted in the cylinder body 2 so as to be movable in the axial direction of the cylinder body 2. The cylinder body 2 has a port 4 through which the interior of the cylinder body 2 communicates with hydraulic units such as hydraulic pressure control modules. The cylinder body 2 carries a reservoir tank 5. The piston 3 carries a center valve 20 at the front (inner) end thereof. When a brake pedal, which is not shown and connected to the piston 3 through a pushrod 6, is depressed and released, the piston 3 moves forward (leftwardly in the figure) and backward. When the piston 3 moves leftwardly in the figure, the center valve 20 closes, thereby producing hydraulic pressure in the master cylinder pressure chamber. The hydraulic pressure produced in the master cylinder pressure chamber is supplied to wheel cylinders or hydraulic units. When the piston 3 moves rightwardly in the figure, the center valve 20 opens, allowing hydraulic fluid in the master cylinder pressure chamber to return to the reservoir tank 5.
FIGS. 9 and 10 show a conventional center valve 120 which corresponds to the valve 20 according to this invention. The valve 120 includes a valve casing 121 provided at the front end of the piston 3 and formed with a passage 122 for hydraulic fluid that extends in the axial direction of the cylinder body 2. A valve seat 130 and a valve body member 140 are mounted in the passage 122. The valve seat 130 is fitted in the passage 122 in abutment with its end wall, and comprises a core 132 and an elastic tube 131 fitted around the core 132. The tube 131 comprises a front end 131a adapted to be pressed against the rear end face 141a of a substantially truncated conical valve body 141 of the valve body member 140, and the remaining cylindrical body 131b. The valve body member 140 includes a shaft 142 extending through the core 132 of the valve seat 130 so as to be slidable in the axial direction of the cylinder body 2 relative to the core 132, and carrying the valve body 141 at its front end.
When the valve body 141 is pressed against the front end 131a of the elastic tube 131, the front end 131a tends to be pushed radially inwardly while being sandwiched between the front end of the core 132, which is made of a metal, and the rear end 141a of the valve body 141 as shown in FIG. 10. When the valve body 141 separates from the tube 131, the tube 131 elastically returns to the original position shown in FIG. 9. When this is repeated, the radially inner portion 131c of the front end 131a of the tube 131 tends to get torn or otherwise damaged. This of course greatly impairs sealability of the valve 120.
Even while the brake pedal is not being depressed and the piston 3 is in the initial position (position of FIG. 1), hydraulic pressure may be supplied into the pressure chamber from hydraulic units for e.g. vehicle stability control (VSC). This creates a pressure difference between the pressure chamber and the chamber communicating with the reservoir 5 through the port 5a. Due to this pressure difference, the front end 131a of the elastic tube 131 is pulled radially inwardly and pressed hard against the valve body 141. Thus, in this situation, too, the radially inner end 131c of the front end 131a of the elastic tube 131 tends to get torn by being repeatedly trapped between the front end face of the core 132 and the rear end face of the valve body 141.
To prevent the elastic tube 131 from getting torn, unexamined Japanese patent publication 2002-302029 proposes to form an annular groove 151 in the front surface of the radially inner portion of the front end of the tube 131 as shown in FIG. 11. When this portion of the tube 131 is pressed against the valve body 141, the groove 151 absorbs elastic deformation of the tube 131 as shown in FIG. 12, thereby preventing the tube from getting torn.
But if FIG. 12 is compared with FIG. 10, it will be apparent that the annular groove 151 scarcely serves to reduce the amount of the radially inwardly stretched portion of the tube when the valve body is pressed against the valve seat or due to a pressure difference between the pressure chamber and the chamber communicating with the reservoir. Thus, the groove 151 cannot effectively prevent the radially inner portion of the front end of the elastic tube from getting torn. By increasing the size of the groove 151, it will be possible to lower the possibility of tearing of the elastic tube to some extent. But with increase in the size of the groove 151, the volume of the front end of the elastic tube will correspondingly decrease. This deteriorates sealability and durability. Thus, the size of the annular groove 151 is limited.
An object of the present invention is to provide a center valve of the abovementioned type which can effectively prevent the elastic tube from getting torn.